Yamaha YZF-R125 Service Manual: Ecu self-diagnostic function

The ECU is equipped with a self-diagnostic function in order to ensure that the fuel injection system is operating normally. If this function detects a malfunction in the system, it immediately operates the engine under substitute characteristics and illuminates the engine trouble warning light to alert the rider that a malfunction has occurred in the system. Once a malfunction has been detected, a fault code is stored in the memory of the ECU.

  • To inform the rider that the fuel injection system is not functioning, the engine trouble warning light flashes when the start switch is being pushed to start the engine.
  • If a malfunction is detected in the system by the self-diagnostic function, the ECU provides an appropriate substitute characteristic operation, and alerts the rider of the detected malfunction by illuminating the engine trouble warning light.
  • After the engine has been stopped, the lowest fault code number is indicated by the engine trouble warning light (or displayed on the FI diagnostic tool). It remains stored in the memory of the ECU until it is deleted.

1. Engine trouble warning light

Engine trouble warning light fault code indication

Digit of 10: Cycles of 1 sec. on and 1.5 sec. off.

Digit of 1: Cycles of 0.5 sec. on and 0.5 sec. off.

Example: 42


a. Light on

b. Light off

c. 1

d. 1.5

e. 0.5

f. 3

Engine trouble warning light indication and fuel injection system operation


* The warning light flashes when any one of the conditions listed below is present and the start switch is pushed:

19: Blue/yellow ECU lead (broken or disconnected)

30: Lean angle sensor (latch up detected)

33: Faulty ignition

39: Fuel injector (open or short circuit)

41: Lean angle sensor (open or short circuit)

50: ECU internal malfunction (memory check error)

Checking the engine trouble warning light

The engine trouble warning light comes on for 3 seconds after the main switch has been set to "ON". If the warning light does not come on under these conditions, the warning light (LED) may be defective.


a. Main switch "OFF"

b. Main switch "ON"

c. Engine trouble warning light off

d. Engine trouble warning light on for 3 seconds

    Circuit diagram
    2. Crankshaft position sensor 4. Main fuse 5. Main switch 6. Radiator fan motor fuse 8. Sidestand switch 9. Battery 17.Engine stop switch 19.Intake air pressure sensor 20.Intake air temper ...

    Self-diagnostic function table
    If the ECU detects an abnormal signal from a sensor while the vehicle is being driven, the ECU illuminates the engine trouble warning light and provides the engine with alternate operating instru ...

    Other materials:

    Installing the starter clutch
    1. Install: Starter clutch assembly Starter clutch bolts "1" NOTE: While holding the generator rotor "2" with the sheave holder "3", tighten the starter clutch bolts. Do not allow the sheave holder to touch the projection on the generator rotor. Stake the end "a" of each start ...

    Checking the radiator
    1. Check: Radiator fins obstruction clean. Apply compressed air to the rear of the radiator. Damage  repair or replace. NOTE: Straighten any flattened fins with a thin, flat-head screwdriver. 2. Check: Radiator hoses Cracks/damage Replace. 3. Measure: Radiator cap ...

    Checking the clutch housing
    1. Check: Clutch housing dogs "1" Damage/pitting/wear  Deburr the clutch housing dogs or replace the clutch housing. NOTE: Pitting on the clutch housing dogs will cause erratic clutch operation. 2. Check: Bearing Damage/wear Replace the bearing and clutch housin ...

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